Alex, you're very right, cars get renewed about every 5 years, which is why there is such an abundance of them available.
My wife has a friend who lives in Japan and my wife was in Japan for awhile, and they both told me the same thing.
They don't like to drive anything too old, so they get rid of them in order to get something newer.
One of the reasons why ppl in Japan tend to renew their vehicles within a few years is that, all cars in Japan have to pass the Automobile Safty Inspection every year. The inspection is very stirict, unlike the vehicle emmision test in US once every 2 yrs.
Certain parts of their cars need to be renewed in order to pass the inspection. (I.e. belts, hoses, valves, boots, pipes, tires & brakes etc.) It's so strict that certain aftermarket parts (i.e. exhausts, headlight Kelvins, % of widow films, suspensions, etc) cannot be permitted either.
IIRC, ppl w/ older cars (say, more than 5yrs) need to pay higher fees / tax for the inspection. In the long run, majority of ppl over there decide to get a brand new car under its warrenties before they'll end up spending a lot of money to pass the test evey year. In addition, the owners of "Eco-cars" (i.e. hybrid vehicles), who bought them new in 2009 ~2012, can be taxed less / subsidized.
With that said, there are PLENTY of quality used cars in Japan. Some even call Japan as "the heaven of used cars". For an instance, affording a "highly tuned-up" '02 Toyota Supra / R33 GTR would be $5,000 ~ $10,000. And, those kind of cars are everywhere..
Up here, we just keep our cars maintained / fixed, pretty much whenever we feel like having it done. Out there, it's mandatory by its running distance / dates. To me, it's more like they have to renew their cars to live economically. And I kind of feel sorry for those car enthusiasts in Japan that they have to spend lots of money for them to simply own their cars.